1. Field of the Invention
The invention relates to an optically active compound having a polymerizable group, a polymerizable liquid crystal composition including the optically active compound, a polymer formed by polymerization of the polymerizable liquid crystal composition and an article containing the polymerizable liquid crystal composition or the polymer.
2. Related Art
In recent years, a polymerizable liquid crystal composition has been utilized to an optical device. When light is irradiated to an optical device, the optical device reflects circularly polarized light in a specific wavelength range depending on the rotational direction of the helix and the length of the helical pitch which are formed by liquid crystal molecules (circularly polarized light separating function).
Examples of the utilization of selective reflection of light include an optical compensator, adornment design and a color filter used for a liquid crystal display device. Furthermore, it is possible to apply to anti-counterfeit detection by utilizing characteristics in which reflected light and transmitted light have unique metallic shine, color tone varies depending on a viewing angle, and such optical characteristics are not exhibited in a printed matter obtained by use of copy machine. In an application of such circularly polarized light separating function, a brightness enhancement film has been proposed, which a quarter-wave plate and an optically anisotropic film that exhibits circularly polarized light separating function are laminated. The brightness enhancement film is expected to exhibit circularly polarized light separating function in the region of all visible light (range at wavelengths of approximately 350 nm to approximately 750 nm). In such a case, it is reasonable to laminate a plurality of layers that have different helical pitches, or to change the helical pitch successively in the direction of the film thickness in order to form this film (the non-patent document 1).
An optical compensation film is used, for example, for a STN (super twisted nematic)-type liquid crystal display (the patent document 1). A film in which a cholesteric phase is fixed, where visible light is passed through and ultraviolet light with wavelengths of approximately 350 nm or less is reflected, is called a negative C-plate, which is used as an optical compensator suitable for an improvement of viewing angle characteristics in a display device having a type such as VA (vertically aligned), TN (twisted nematic), OCB (optically compensated birefringence) or HAN (hybrid aligned nematic). A combination of the negative C-plate with an optical compensation layer such as a positive A-plate having positive birefringence makes it possible to use for the viewing angle compensation in a display device having a VA type (the patent document 2).
Development of a polymerizable cholesteric liquid crystal composition has been expected, where in any use described above, the liquid crystal composition has a cholesteric phase at room temperature, exhibits an excellent orientation and has a fast polymerization rate on irradiation with ultraviolet light, and a polymer formed by curing has a suitable optical anisotropy (Δn), has transparency, and is excellent in thermal resistance and moisture resistance.
The cholesteric liquid crystal composition can be usually prepared by the addition of an optically active compound (a chiral agent) to a nematic liquid crystal composition. The liquid crystal molecules are required to have a helical structure in which the helical pitch is quite short, in order to reflect circularly polarized light in the range of ultraviolet light to visible light. Then, the use of an optically active compound having a large helical twisting power (HTP) is required. In this case, when an optically active compound having a small HTP is used, its additive amount should be increased, and thus it is hard to adjust the other characteristics, especially the temperature range of a cholesteric phase and the wavelength range of selective reflection. The optically active compound has no liquid crystallinity in many cases, and the liquid crystallinity of the composition may be disappeared when additive amount of the optically active compound is increased and the desired cholesteric phase may not be exhibited.
When an optical film is produced, a polymerizable chiral agent is added to a polymerizable liquid crystal compound, forming a polymer, and fixing a helical structure that is formed by the liquid crystal molecules. Such a chiral agent is required to have characteristics such as a large HTP, a low temperature dependence of HTP, an excellent compatibility with a liquid crystal compound and an excellent long term preservation stability (not easily be crystallized). There may be cases where a chiral agent having a large HTP becomes quite expensive and thus unsuitable for an industrial use when the chiral agent has a complex structure and thus it is difficult to be synthesized. Comprehensive characteristics such as a fast polymerization rate, a high mechanical strength, excellent coating properties, an excellent solubility, a high degree of crystallinity, a low shrinkage, a low water permeability, a low water absorptivity, a low gas permeability, a suitable melting point, a high glass transition temperature, a suitable clearing point, a high chemical resistance, a high thermal resistance, a high weather resistance and a small photoelasticity are required for the polymer.
Polymerizable and optically active compounds are disclosed in JP H09-020781 A (1997) (the patent document 3), JP 2004-504285 A (the patent document 4), JP 2007-269640 A (the patent document 5) and JP 2009-084178 A (the patent document 6). Now, the development of an optically active compound, which has an excellent compatibility with a liquid crystal compound, an excellent solubility in an organic solvent or the like, and a high polymerization rate during the polymerization is desired.